It is previously known that to enable coherent demodulation of acknowledgement (ACK/NACK) signals from multiple users in the uplink of a communication system, a demodulation reference signal (DRS) is allocated to each of the user equipments (UE). The DRS is taken from a set of orthogonal DRS.
Furthermore, in the downlink, a broadcast channel is used for transmission of control information to the UEs from a base station. The information in this broadcast channel is composed of multiple segments of information, denoted control channel elements (CCE). Each UE is allocated with one or several consecutive CCEs and receives its dedicated control information, denoted physical downlink control channel (PDCCH) in its allocated CCE segments. The number of allocated CCEs per PDCCH is 1, 2, 4 or 8.
The PDCCH contains information about where and in which format a downlink data burst, denoted physical downlink shared channel (PDSCH), or an uplink data burst, denoted physical uplink shared channel (PUSCH) is transmitted. The UE first finds and reads the PDCCH, then receives the PDSCH and decode its message. At a later point in time, the UE sends an ACK/NACK in response to the received PDSCH message in the uplink to the base station.
To avoid explicit scheduling of a resource index (DRS indices or ACK/NACK indices) to be used for this ACK/NACK message, it is implicitly indicated from the location of the PDCCH which contained information about the decoded PDSCH. More precisely, the location (index) of the first CCE in the PDCCH uniquely determines the resource index to be used for the uplink ACK/NACK transmission.